Aliphatic polycarbonates (APCs) have utility as polyol building blocks for the construction of co-polymers such as flexible urethane foams, urethane coatings, rigid urethane foams, urethane/urea elastomers and plastics, adhesives, polymeric coatings and surfactants among others. Examples of such APCs include poly(propylene carbonate) (PPC); poly(ethylene carbonate) (PEC); poly(butylene carbonate) (PBC); and poly(cyclohexene carbonate) (PCHC) as well as copolymers of two or more of these.
To have utility in these applications, it is preferable that all polycarbonate polymer chain ends terminate with hydroxyl groups. Such hydroxyl groups serve as reactive moieties for cross-linking reactions or act as sites on which other blocks of a co-polymer can be constructed. It is problematic if a portion of the chain ends on the APC are not hydroxy groups since this results in incomplete cross-linking or termination of the block copolymer. A typical specification for aliphatic polycarbonate polyol resins for use in such applications is that at least 98% or in some cases greater than 99% of chain ends terminate in hydroxyl groups. In addition, these applications typically call for relatively low molecular weight oligomers (e.g. polymers having average molecular weight numbers (Mn) between about 500 and about 15,000 g/mol). It is also desirable that the polyols have a narrowly defined molecular weight distribution—for example, a polydispersity index less than about 2 is desirable, but much narrower distributions (i.e. PDI<1.2) can be advantageous. Furthermore, for certain applications, polyol polycarbonates having little or no contamination with ether linkages are desirable.
Aliphatic polycarbonates can be conveniently synthesized by copolymerization of carbon dioxide and epoxides as shown in Scheme 1.

Currently, there are several catalytic systems utilized for such syntheses, namely: heterogeneous catalyst systems based on zinc or aluminum salts; double metal cyanide (DMC) catalysts; and homogenous catalysts based on coordination complexes of transition metals or aluminum.
The catalytic systems using heterogeneous zinc or aluminum salts are typified by those first described by Inoue in the 1960s (for example in U.S. Pat. Nos. 3,900,424 and 3,953,383. Further improvements to these catalysts have been made over the years (for example as described in W. Kuran, et al. Chem. Macromol. Chem. Phys. 1976, 177, pp 11-20 and Gorecki, et al. J. Polym. Sci. Part C 1985, 23, pp. 299-304). Nonetheless, these catalyst systems are generally not suitable for producing polyol resins with the low molecular weights and narrow polydispersity demanded by many applications. The catalysts are of relatively low activity and produce high molecular weight polymer with broad polydispersity. Additionally, the polycarbonates produced by these catalysts have a significant proportion of ether linkages in the chain which can be undesirable in certain applications.
A second class of catalysts for the polymerization of epoxides and CO2 are the double metal cyanide (DMC) catalysts. Such catalysts are exemplified by those reported by Kruper and Smart in U.S. Pat. No. 4,500,704. Compared to the Inoue-type catalysts, the DMC systems are better suited to the formation of low molecular weight polymers and produce a predominance of chains with hydroxyl end groups. However, these catalysts produce polymers having a high proportion of ether linkages and the materials they produce are more properly regarded as polycarbonate-polyether copolymers rather than as aliphatic polycarbonates per se.
A more recently developed class of catalysts is based on coordination complexes of aluminum or a variety of transition metals, particularly complexes of cobalt, chromium and manganese. Examples of such catalysts are disclosed in U.S. Pat. Nos. 6,870,004 and 7,304,172. In some cases these catalytic systems are highly active and are capable of providing aliphatic polycarbonate with narrow polydispersity, a high percentage of carbonate linkages and good regioselectivity (e.g. high head-to-tail ratios for incorporation of monosubstituted epoxides). However, at high conversions under standard conditions, these catalysts produce high molecular weight polymers that are not suitable for many polyol applications. Additionally, using these systems, it has not been practical to synthesize polycarbonate polyols having a high percentage of hydroxyl end-groups.
The lack of hydroxyl end-groups is due to the fact that anion(s) associated with the metal center of the catalyst complex become covalently bound to the polymer chain during initiation of polymer chain growth. This is true also of anions associated with any optionally present cationic co-catalysts used in these reactions. Without wishing to be bound by theory or thereby limit the scope of the present invention, the sequence shown in Scheme 2, depicts a probable reaction sequence showing why the anions (denoted —X) associated with the catalyst complex
become covalently linked to the polycarbonate chain.

The counterions —X typically used for these catalysts include halides, sulfonates, phenolates, carboxylates and azide. Because polymerization is initiated when one of these anions opens an epoxide ring, one end of each polymer chain (the initiation end) is necessarily capped with a non-hydroxyl moiety such as a halogen, an alkylsulfonate, a phenylether, an acyl group, or an azide, respectively.
The other factor disfavoring the use of these catalytic systems to produce polyol resins is the fact that they produce high molecular weight polymer when taken to high conversions. Typical molecular weights are in the range of 20,000 to 400,000 g/mol-values well above the molecular weight range desired for most polyol resin applications. Potential strategies to produce lower molecular weight materials include: stopping the polymerization at low conversion; using high catalyst concentrations; degrading the high molecular weight polymer to shorter chains, or using chain transfer agents (CTAs) such as alcohols during the polymerization. Stopping the reaction at low conversion or increasing the catalyst concentration are undesirable due to cost considerations and added difficulties in purification occasioned by the increased concentration of catalyst-derived contaminants in the crude polymer. Degradation of higher molecular weight polymers to produce low molecular weight resins leads to increased polydispersity, adds additional steps to the production process, and leads to contamination with cyclic by-products. Chain transfer agents can be successfully employed to lower the molecular weight of the polymer without a significant increase in cost or contamination. However, this strategy does not alleviate the problem of non-hydroxyl end groups since polymer chains initiated by chain transfer agent will still have one end capped with a non-hydroxyl moiety (i.e. an ether corresponding to the alcohol used as the CTA).
As such, there remains a need for catalysts and methods that are capable of efficiently producing polycarbonate polyols having high carbonate content.